The invention relates to a method of carrying out an electro-chemical treatment, especially, for electro-chemically coating conducting parts or parts made to be conducting. The parts are placed in a container which is filled with an electrolyte and includes two electrodes (anode, cathode) connected to a constant voltage source. Electro-chemical coating, i.e. galvanic coating, constitutes the major application of the invention. However, provided the anode and cathode are suitably exchanged, it is also possible to use the method for electro-chemical cleaning or electro-chemical abrasion. Furthermore, anodic/cathodic immersion painting may be included by the invention. In addition, the invention relates to a plant for carrying out an electro-chemical treatment, especially for electro-chemically coating conducting parts or parts made to be conducting. The plant includes a container filled with an electrolyte with two electrodes (anode, cathode) in the container connected to a constant voltage source.
Metallic or plastic parts with surfaces that are pre-treated to render them conductive are galvanically plated for corrosion protection purposes and partly, for decorative purposes. Depending on the size, shape and number of parts or products to be plated, different process techniques are applied.
In the case of continuous processes, endless belts, tubes or wires are pulled through a galvanic bath at a speed of 10 to 300 m/min. Contact of the cathode is established by rollers. The higher the speed, the higher the current density to be applied. In the case of zinc plating, up to 200 A/dm2 can be achieved. This produces a plating thickness of 15 xcexcm which takes about 17 seconds.
In the case of a rack method, parts are placed on to the rack, which is electrically connected to the cathode, and suspended in the galvanic bath. For zinc plating, the current density ranges between 2 to 4 A/dm2. A plating thickness of 15 xcexcm builds up in about 20 to 40 minutes. The rack method is suitable for very large parts, for example tubes several meters long and for small parts, for instance valuable turned parts. In general, the parts are placed on the rack manually, since the rack method is not suitable for mass production.
Articles in bulk, especially articles such as bolts, nuts, washers and the like are plated by a drum method. The parts are placed into a perforated drum which is immersed in the galvanic bath. Inside the slowly rotating plastic drum, flexible, isolated cables with non-insulated ends, move over the parts to provide the electric contact with the cathode. In the case of zinc plating, the current density ranges between 0.5 to 1.5 A/dm2. This produces a plating thickness of 15 xcexcm in about 60 to 160 minutes.
Methods and devices for surface coating are known from DE 31 21 397 C1 and DE 32 30 108 C2. Here, electro-chemical surface coating of small parts is shown. The parts are received in a drum which is rotatably drivable in a container. In a first axis position during the coating phase, coats the parts are coated at a low rotational speed. In a second vertically oriented axis position, after the treatment fluid has been drained off, the parts are centrifuged at an increased rotational speed. The means used to carry out the electro-chemical process are not explained in greater detail in these publications.
In continuous operating plants, rack and drum plants, electro-chemical surface treatment takes place in open baths. As a rule, if several such baths are arranged side by side, they form a considerable bath surface. While the processes take place, spray mist and vapors occur which constitute workplace pollution. Accordingly, considerable measures are taken to ensure extraction of the spray mist, vapors and gases which occur during the various process stages. Even in the case of smaller systems, exhaust quantities ranging between 5000 and 10,000 m3/h have to be dealt with; in the case of larger systems, exhaust quantities ranging between 100,000 and 200,000 m3/h may have to be extracted and treated. The exhaust air enters an air washer and is thereafter released into the open atmosphere. Corresponding quantities of fresh air have to be introduced from the outside, so that considerable ventilator capacities are provided. In the winter, sucked-in cold fresh air has to be heated which requires large amounts of energy, which, in turn, leads to the need for heat exchangers through which hot exhaust air is conducted in a counter flow to cold fresh air.
It is an object of the present invention to provide a method and device of the above-mentioned type which, while having a simple design, achieves a high coating output.
The objective is achieved by a method where the parts are connected cathodically by a hub of the basket. The electrolyte is pumped through the container in a cycle. The container is sealed to remain gas-proof.
Because the parts are connected cathodically by the hub of the basket, the power supply to the parts is ensured at all times. Circulating the electrolytic solution through the container ensures that the coating is applied to the parts in a uniform and defect-free way. In a preferred embodiment, the parts are re-arranged during the coating operation by rotating the basket around a horizontal axis.
In the container, preferably a flow speed of the electrolytic solution is maintained at least 1 m/min; especially approximately 10 m/min. It is possible to achieve high currency densities which lead to short coating times. The current density is preferably set to approximately 10 A/dm2 in the case of zinc electrolytes and aluminum electrolytes and to approximately 25 A/dm2 in the case of acidious copper electrolytes. In particular, an electrolytic solution temperature advantageous for the process is maintained in the container. Optionally, the electrolytic solution has to be heated or cooled within the closed cycle in a suitable place. In the case of non-aqueous electrolyte systems, the term xe2x80x9celectrolytic solutionxe2x80x9d also includes salt melts.
A compensating container in the cycle for the electrolytic solution can ensure permanent freedom from gas in the container.
After a coating phase, the electrolytic solution is pumped out of the container. Remaining electrolyte is centrifuged off the surface of the parts under the effect of a centrifugal force. For this purpose, the basket axis is preferably first set to a vertical position.
This process can be followed by a washing operation in the container itself. Any water adhering to the parts is also centrifuged off the parts under the effect of a centrifugal force. To achieve a uniform coating, it is particularly advantageous if, during the electro-chemical treatment, the parts are continuously re-arranged during the electro-chemical treatment in the stream of electrolytic solution.
To continue to improve the process when using aqueous electrolytes, it is proposed that, a principle-related H2-containing partial stream of the electrolytic solution (catholyte) is extracted in the vicinity of the parts, and a principle-related O2-containing partial stream of the electrolytic solution (anolyte) is extracted in the vicinity of the anode during the coating phase. Accordingly, through-mixing is avoided and it ensures that in the vicinity of the parts, an electrolyte flow with a sufficiently high percentage of metal ions is added. To carry out the process economically and especially to recover part of the energy used for water decomposition purposes, an inert anode is used. The catholyte stream outside the container, while forming additional H2, is fed with metal ions or metal ion complexes. The anolyte stream and the catholyte stream, especially if enriched with metal ions or metal iron complexes, are fed into the cathode chamber or, respectively, the anode chamber of a fuel cell.
For aprotic (proton-free/non-aqueous) electrolytes, and for aqueous electrolytes with very high current requirements, it is advisable to transport the catholyte and anolyte separately. This ensures the transport of the largest possible quantity of material.
Furthermore, it is the object of the invention to provide a method and a plant where the energy balance of the electro-chemical treatment of parts is improved. Preferably, the environmental balance is also more advantageous than in the case of prior art methods and plants.
The objective is achieved by a method where the electrolyte flows through the container. The decomposition products of the water at the electrodes, i.e. H2 and O2 are extracted separately from the electrolyte. The decomposition products are fed into a H2/O2 fuel cell for degassing the electrolyte and recovering energy. Accordingly, the percentage of energy used for the decomposition of water from the electrolytic solution, which decomposition takes place at the electrodes, can be recovered to a considerable extent or almost entirely.
According to a preferred method of operation, in a metal dissolving reactor, metal ions are added to the catholyte. While additional H2 is formed, the O2 excess which occurred during electro-chemical coating is compensated for completely. In this way, the fuel cell can be operated in an optimum way. With such complete combustion, up to 30% of the energy used for electro-chemical treatment can be recovered. This constitutes a considerable advantage with ever increasing energy prices. Thus, the additional expenditure required for the plant is amortized within an acceptable period of time. Since the gas irritants, cold-combusted H2 together with the O2 are eliminated, improved workplace conditions are achieved. In this embodiment it is also possible to guide the electrolytic solution in a completely closed cycle. Thus, the solution streams leaving the fuel cell can be returned into the container. In each case, the electrolytic solution has to be newly chemically determined. In particular, a metal dissolving process has to be integrated into the cycle. If, in an especially advantageous embodiment, the closed cycle takes place under the exclusion of air, the workplace values are further improved. Also, the considerable plant expenditure for extracting and washing air becomes largely superfluous.
In the container, a flow speed of the electrolytic solution of at least 10 m/min preferably has to be maintained. Accordingly, it is possible to achieve high current densities which lead to short coating periods. The current density is preferably set to at least 4 A/dm2 in the case of a zinc electrolytic solution and to at least 10 A/dm2 in the case of an acid copper electrolytic solution. The temperature for the electrolytic solution is also maintained, in the container so as to be advantageous to carry out the process. Optionally, the electrolytic solution is heated or re-cooled in the closed cycle in a suitable place.
The H2 and O2 gases fed separately into the fuel cell are advantageously extracted from the container directly in the place where they occur. H2 is extracted together with the catholyte stream near the cathode. O2 is extracted together with the anolyte stream near the anode. The catholyte stream can be fed into the anode chamber of the fuel cell. The anolyte stream can be fed into the cathode chamber of the fuel cell, without the need for any further separating measures in either case.
In order to produce identical quantities of H2 and O2, so that complete cold combustion can take place in the fuel cell, metal ions or metal ion complexes are added to the catholyte stream in a metal dissolving reactor, to form additional H2.
The solution streams separately leave the chambers of the fuel cell. The streams are combined behind the fuel cell. After being analyzed and chemically re-determined in a compensating container, the streams are again fed into the container in the form of an electrolytic solution.
In a preferred way, the treatment container is emptied after completion of the electro-chemical treatment. Any electrolytic solution adhering to the parts is centrifuged off the parts under the influence of a centrifugal force. This operation can be followed by a washing operation in the container itself. Thereafter, any water adhering to the parts is centrifuged off the parts under the influence of a centrifugal force. To achieve a uniform coating, it is advantageous if, while being electro-chemically treated, the parts are continuously re-arranged in the stream of the electrolytic solution.
Furthermore, the initially mentioned objective is achieved by a device for carrying out an electro-chemical treatment. The device includes a hub of the basket in the form of a cathode. The container includes inflow and outflow means. Means for controlling the electrolytic cycle is connected with the inflow and outflow means. The container can be sealed so as to be gas-proof.
In the container, a rotatable basket receives the parts to be coated and continuously re-arranges the parts by the rotating basket around a horizontal axis during the coating operation. Furthermore, electrolytic fluid is continuously pumped and circulated through the container in a closed cycle. The electrolytic fluid is continuously reprocessed outside the container. Accordingly, it becomes possible to increase the current density while avoiding non-uniform coatings.
In a preferred embodiment, the anode is arranged half-cylinder-like parallel to the basket axis underneath the basket. Inflow means for the electrolytic solution are arranged between the basket surface and the anode.
To further improve the process, one outflow aperture for the catholyte is arranged inside the basket. At least one outflow aperture for the anolyte is positioned directly at the anode outside the basket. In particular, with reference to the axis of the drum, the at least one outflow aperture for the anolyte is positioned radially outside the anode in the container. The outflow apertures for the discharge of anolyte can be distributed over a half-cylinder surface at the container.
To achieve the considerable electric currents, a shaft journal at the basket leads through the housing and serves as a current conductor. The basket can be provided with an outer perforated electrically non-conductive cylindrical casing and an inner perforated highly conductive hollow hub. Again, the interior of the hollow hub, in respect of flow, is openly connected to a co-axial hollow journal penetrating the housing in order to extract any electrolytic solution which has flowed past the parts. A plurality of perforated tubular members are provided to supply the electrolyte solution. The tubular members are distributed over a half-cylinder surface and extend parallel to the axis of the basket. Optionally, one double-walled perforated half-cylinder is supplied through an end wall of the container.
A solution for the further above-mentioned objective comprises a plant to carry out an electro-chemical treatment. The plant includes a supply line to supply the electrolyte to the container. Two separate extraction lines extract the anolyte and catholyte from the container. A H2/O2 fuel cell with supply lines leading to an anode chamber and to a cathode chamber are connected to the extraction lines of the catholyte and anolyte, respectively. The plant parts which are briefly described here and whose further preferred embodiment will be described below, enable an electro-chemical treatment which, in turn, allows the above-described improvement in the energy balance and workplace values.
To achieve a closed electrolytic solution cycle, two separate lines exit from the anode chamber and from the cathode chamber of the fuel cell are combined and connected to the supply line to take electrolytic solution to the container. A metal dissolving reactor is arranged in the pipeline cycle for electrolytic solution. Especially, it is arranged in the line for the catholyte, behind the container. At the same time, the anode in the container is preferably an inert metal.
A H2/O2 fuel cell is provided in the form of a plate and frame cell. Accordingly, the size of the fuel cell can easily be adapted to the required capacity. The anodes and cathodes include of a catalytically coated material. The cell interior is divided by an ion exchanging membrane. The low exchange membrane forms the (cathodically switched) cathode chamber and the (anodically switched) anode chamber.
The container with the rotatably supported basket and the elements fixedly arranged therein is preferably pivotable in its entirety around a horizontal axis by 90xc2x0. A driving motor is coupled to the basket. The driving motor can be switched to a low speed when the basket axis extends in the horizontal direction to re-arrange the parts. When the basket axis extends in the vertical direction, the motor can be switched to a higher speed to centrifuge the parts.
As a result, the rotatable basket can be pivoted inside the container or together with the container from a horizontal axis position into a vertical axis position. Due to this measure it is possible, during treatment, to re-arrange the parts in the basket without having to load the basket into a different container, and subsequently to centrifuge the parts after the electrolytic solution has been pumped out of the container. Accordingly, it is possible to reduce the amount of electrolytic solution removed together with the parts which, at a later stage, have to be taken out of the container.
To further reduce the amount of electrolyte to be removed, it is possible, subsequently, to carry out washing operations in the rotating basket inside the container. Washing fluid is introduced into the container and then pumped off. The parts subsequently are centrifuged together with the basket.
The greatest economic benefit of the inventive method and of the inventive device will probably be derived by the zinc plating process to which reference will be made below. A zinc plating plant with a device in accordance with the invention shortens throughput times, saves energy and space, reduces re-loading operations for the parts and minimizes waste water and other waste.
A treatment cell, in this case, is a pivotable galvanic container in which the parts are electrolytically coated. To be able to achieve the high current densities required, the electrolytic solution has to flow through the parts and the anode at a high speed. The hydrogen developing at the cathodically switched parts and the oxygen developing at the anode are extracted together with the respective stream of electrolytic solution.
The catholyte stream contains finely distributed hydrogen gas and is depleted in respect of zinc. To increase the zinc content, the catholyte stream is conducted through a zinc dissolving reactor which is fed with metallic zinc, with additional hydrogen developing. From the reactor, the catholyte stream is guided into the anode chamber of the H2/O2 fuel cell. The gaseous hydrogen is dissolved under oxidation. The anolyte stream is guided directly into the cathode chamber of the H2/O2 fuel cell where the gaseous oxygen is dissolved under reduction. The two electrolytic solution streams, which are gas-free and low in gas, respectively, flow out of the fuel cell and are combined and returned into the coating cell. Thus, the fluid system is closed. After completion of the coating operation or following the coating phase, the coating cell is pivoted by 90xc2x0 into a position where the basket axis extends in the vertical direction. The electrolytic solution is pumped off and the solution residue is centrifuged off the parts by driving the basket at an increased rotational speed of approximately 300 rpm. In subsequent treatment stages, water for rinsing purposes or other treatment media can be introduced into the coating cell and subsequently pumped off. Optionally, the parts can be circulated with a horizontal basket axis. This optional operation is, in any case, followed by a centrifuging operation at an increased speed, with a vertical basket axis. Thereafter, the parts are removed from the coating cell with the basket, in a vertical axis position, by lifting out the basket from the coating cell.
In a practical application, the basket can have an inner diameter of 250 mm. The basket hollow hub from which catholyte is extracted has a diameter of 100 mm. The basket can have a height of 300 mm. The resulting volume is approximately 12 liters. The volume can be filled up to one third with parts.
If the parts are metric bolts M8xc3x9725 for example, the resulting bulk weight is 4 kg/l and the resulting specific surface area is 12 dm2/kg. In consequence, if a quantity of the type of bolts has been filled into the basket, a surface area of approximately 200 dm2 is obtained. To achieve a current density of 10 A/dm2, a rectifier capacity of at least 2000 A is required. If the batch size were to be increased to 100 to 200 kg, capacities of 12,000 to 24,000 A would be needed.
With a current density of 10 A/dm2 the coating time is only 4 to 6 minutes. Because of the high load per liter, the ratio of quantity of electricity to volume of electrolyte, an increase in the temperature of the electrolyte occurs. This is advantageous for the separation rate and the electricity exploitation. Care has to be taken to ensure that the additives used when determining the composition of the electrolytic solution function in the required way at such temperatures. If necessary, the electrolyte may be heated or cooled.
According to a special example, the coating unit is filled with an aqueous zinc electrolyte with the following composition:
20 g/l zinc
250 g/l KOH
50 g/l K2CO3 
10 ml/l SurTec 704 I (commercial additive)
1 ml/l SurTec 704 II (commercial additive)
10 ml/l SurTec 704 R (commercial additive)
5 ml/l SurTec 701 (commercial additive).
The basket is filled with 12 kg of metric steel bolts M8xc3x9725. As described, the bolts are cathodically zinc-coated in the coating unit for 6 minutes at 10 A/dm2, whereupon the bolts comprise an average zinc coating thickness of 13 xcexcm.
To achieve the high separation rate, an extremely good electrolyte convection in the vicinity of the part surface constitutes an essential factor. The electrolyte convection is ensured by re-arranging the parts as a result of circulating the parts in the basket and by means of setting uniform inflow and outflow conditions in the coating cell.
The inert anode is provided in the form a catalytically coated anode in order to ensure the highest possible anodic current densities. The half-cylinder-shaped anode is perforated. The electrolyte flows through from the inside and outside at a high flow speed inside the coating cell.
In a zinc dissolving reactor, metallic zinc in the alkaline electrolytic solution, while in contact with a catalytically coated material, is dissolved while hydrogen develops.
This process stage is applied to supplement the zinc used in the coating cell. The zinc dissolving reactor provided for this purpose is sealed so as to be airtight towards the outside. The catholyte which, in the form of a partial stream extracted from the interior of the basket after having flowed past the cathodically switched parts, flows through the reactor. As a result of the proceeding, the catholyte is depleted of zinc and enriched with hydrogen gas. The zinc dissolving reactor is supplied with additional zinc, and the hydrogen content is additionally increased. From the zinc dissolving reactor, the catholyte is conducted into the fuel cell. In the course of the continuous operation, at the cathode of the coating cell and in the zinc dissolving reactor, together, there is produced, at any point in time, twice as much hydrogen as there is produced oxygen at the anode of the coating cell. The H2/O2 ratio thus corresponds to the requirements of a complete, residue-free, cold reaction in the H2/O2 fuel cell relative to water (H2O).
In view of the high load per liter, the ratio of the quantity of electricity to the volume of electrolyte, rapid changes occur in the electrolytic solution. The changes are preferably compensated for by fully automatic process control means. The control means monitor and set all major electrolyte parameters. Apart from the conventionally recorded and controlled parameters of temperature, pressure, voltage and current, these parameters are as follows:
According to a further special example, a coating unit modified in such a way that the electrolyte cycle comprises neither a metal dissolving reactor nor a fuel cell is filled with a water-free aprotic aluminum electrolyte with the following composition (at room temperature):
250 g/l AlEt3 (triethylaluminium)
150 g/l AliBut3 (trisobuthylaluminium)
80 g/l KF (potassium fluoride)
in toluene as a solvent. The basket is filled with 12 kg of metric steel bolts M8xc3x9725. The basket is inserted into the coating cell which is then hermetically sealed. The coating cell is first flooded and rinsed with dried nitrogen and argon, respectively. The aluminum electrolyte is pumped into the coating cell. The aluminum electrolyte displaces the nitrogen and argon respectively from the cell. As described, the bolts are cathodically aluminized for 5 minutes at 10 A/dm2. The electrolyte is pumped and centrifuged off. The bolts comprise an average aluminum coating thickness of 15 xcexcm.
The treatment cell (coating cell) is preferably integrated into the overall plant of treatment machines whose individual machines can carry out the following treatment stages for example:
de-oiling
degreasing
pickling
electrolytic cleaning
electrolytic coating
chromating; blue-, yellow- or black-chromating sealing,
with the fourth and fifth stages being carried out by an inventive treatment cell. The basket, which can be lifted out of the treatment cell, has to be designed in such a way that it can be inserted into all the other individual machines of the plant.
If, after each treatment stage, the parts are rinsed in the respective machine and dried by being centrifuged, any carry-over between the treatment operations will be minimized.
From the following detailed description, taken in conjunction with the drawings and subjoined claims, other objects and advantages of the present invention will become apparent to those skilled in the art.